Automatic headlamp control circuit



y 1960 E. G. MATKINS 2,935,646

AUTOMATIC HEADLAMP CONTROL CIRCUIT Filed June 23. 1958 3 Sheets-Sheet 1IN VEN TOR.

I iaw A T TOR/V5 Y May 3, 1960 E. G. MATKINS AUTOMATIC HEADLAMP. CONTROLCIRCUIT Filed June 23, 1958 3 Sheets-Sheet 2 m w m m ATTORNEY NQ h m w QM u Wk v Q I \N w w W l Q United States Patent AUTOMATIC HEADLAMPCONTROL CIRCUIT Eugene G. Matkins, Anderson, Ind., assignor to GeneralMotors Corporation, Detroit, Mich., a corporation of DelawareApplication June 23, 1958, Serial No. 743,781

17 Claims. (Cl. 31583) This invention relates to automatic controlsystems and more particularly to light responsive systems for automaticcontrol of vehicle headlamps.

Automatic control systems for vehicle headlamps are commonly used forswitching the headlamps from the upper beam to the lower beam when anoncoming vehicle is encountered and vice versa when the vehicle haspassed. In such systems, it is desirable to employ circuit componentswhich may be energized at the low voltage level of the vehicle batteryto obviate the need for a high voltage p'ower supply. In the controlcircuit, this is accomplished by utilizing low voltage electron tubes inconjunction with a transistor as a switching device for a control relay.In order to obtain positive switching action, the amplifier stages areconnected in bistable or flip-flop circuit fashion and regenerativefeedback is derived from the coil of the control relay itself to assurepositive switching action. 1

In the light sensing or pick-up device, low voltage operation isachieved by the use of a photoconductive cell in lieu of the morecommonly used photomultiplier tube which requires high voltageenergization. The photoconductive cell, such as a broad areacadmium-selenide cell, has the disadvantage of a long time constant orslow response to changes in light intensity. In order to achieve thefast response necessary for automatic headlamp control systems, thephotoconductive cell is connectedwith the input circuit of a self-biasedamplifier stage in such a manner that the self-bias voltage change isdelayed with reference to the signal voltage. This is preferablyaccomplished by utilizing a self-biased amplifier stage having acondenser connected in parallel with the cathode resistor to preventrapid change of the cathode voltage. Since this arrangement would permitflashes of light of less intensity than the dimming level to causedimming action, the condenser is switched out by the control relay whenit is in the upper beam position or alternatively, an

additional condenser may be connected across the input circuit from gridto ground when the control relayisin the upper beam position.

In such automatic headlamp control systems, the sensitivity of thesystem is caused to change from a dim sensitivity control value to ahold sensitivity control value when the headlamps are switched fromupper beam to lower beam. This sensitivity change prevents the controlsystem from returning the headlamps to upper beam in response to thedecreased light when the headlamps of an oncoming vehicle are switchedfrom upper to lower beam. In the inventive system, the change insensitivity is obtained either by changing the voltage across thephotoconductive cell or by changing the load resistance of the cell asan incident to operation of the control relay. The overall systemsensitivity is changed by a bias adjustment of the amplifier whichchanges the dim control sensitivity and the hold control sensitivityvalues proportionally.

A more complete understanding of this invention may "ice be had from thedetailed description which follows taken with the accompanying drawingsin which:

Figure 1 is a schematic diagram of one embodiment of the invention; and

Figures 2 and 3 are schematic diagrams showing modifications of thesystem of Figure 1.

Referring now to the drawings, there are shown illustrative embodimentsof the control system especially adapted for automatic control of theheadlamp circuit 10 of an automotive vehicle. In general, the systemcomprises a pick-up unit 12 including a photoconductive cell 14 andwhich is mounted on the vehicle in a suitable location to intercept thelight beam from the headlamps of an oncoming vehicle. An automaticcontrol unit 16 for the vehicle headlamps is connected with the pick-upunit through a connector device 17 and is suitably located within eitherthe passenger or the engine compartment. The control unit 16 comprises adirect coupled amplifier which includes electron tube stages 20 and 22and transistor stage 24. The transistor stage 24 energizes a controlrelay '26 which in turn controls the energization of a power relay 228for switching the headlamp circuit 10'. A manual control unit 18 islocated for convenient operation by the driver of the vehicle. Theentire system is energized directly from a low voltage source or battery30.

The photoconductive cell 14 is suitably of the type known as a broadarea cell which may comprise a photosensitive material ofcadmium-selenide. A suitable broad area cell of this type ismanufactured by the Radio Cor poration of America and identified as typeC-72l8. The photoconductive cell exhibits a variation in electricalresistance in response to incident light and thus a current variationcorresponding to light intensity is developed when a voltage isimpressed across the cell. The supply voltage circuit for the cell 14includes the battery 30 which has one terminal connected to a point ofreference potential or ground and the other terminal connected through aconductor 32, terminal 34, conductor 36, resistor 38, and apotentiometer resistor 40 which is. con nected to ground through asensitivity switching circuit including a conductor 42 and the controlrelay 26. To apply an adjustable value of voltage across the cell 14, amovable contact 44 on potentiometer resistor 40 is connected throughterminal 4-6 to cell 14 and thence through a terminal 48, currentlimiting resistor 50 and a variable load resistor '52 to ground.Accordingly, a signal voltage corresponding to incident light intensityis developed across the load resistor 52 and in the absence of light thesignal voltage is substantially Zero.

The response of the photoconductive cell to light intensity is afunction of the temperature of the cell and it is desirable to stabilizethe temperature of the cell at some value greater than the ambienttemperature. This temperature stabilization is provided by a cell oven54 which suitably surrounds the cell and includes a resistance heaterelement 56 and a temperature responsive switch 58 which are seriallyconnected through a conductor 60 and the conductor 32 to the battery 30.

In the control circuit, a direct-coupled amplifier controls theenergization of the control relay 26. The control relay includes anenergizing coil 88, a pair of fixed contacts 72 and 73, and a movablecontact 74 which is spring biased into engagement with fixed contact 72.The first amplifier stage 20 comprises a triode electron tube adaptedfor low voltage energization and having its plate connected through aplate load resistor 62 to the supply voltage conductor '36. The cathodeis connected to ground through a cathode bias or self-bias resistor 64which is connected in series with a voltage divider resistor 66 acrossthe supply voltage conductor 36 to develop a cathode bias voltage. Acondenser 68 is connectib le in parallel with the cathode resistor 64,through a conductor 7t and contacts 72 and 74 of the control relay 26,and with the cathode resistor 64 formsa time constant circuit for apurpose to be described hereinafter. The bias conditions, i.e., thequiescent value of electrode voltages developed by the aforementionedcircuits, on the amplifier stage 2% are such that it is substantiallynon-conductive in the absence of a signal voltage across resistor 52.

The amplifier stage 22 is also an electron tube adapted for low voltageenergization and the plate is connected through plate load resistor 76to the supply voltage conductor 36. The output voltage from the firstamplifier stage 20 is derived from its plate and applied directly to thegrid of the amplifier stage 22. The cathode of amplifier stage 22 ismaintained at a positive voltage with respect to ground by connectionthrough a conductor 78 and a terminal 80 to the movable contact 82 of avoltage divider 84 which is energized from the supply voltage conductor60. The grid of amplifier stage 22 is connected through resistor 86 andthe energizing coil 88 of control relay 26 to ground. Thus, the plateload resistor 62, resistor 86 and the resistance of coil 88 form avoltage divider which maintains the plate of amplifier stage 20 atapproximately half the supply voltage value. The bias conditions, i.e.,the quiescent value of electrode voltages developed by theaforementioned circuits, on amplifier stage 22 are such that it isconductive due to the plate voltage of stage 20 when the latter isnon-conductive in the absence of light on the cell 14.

The transistor stage 24 suitably comprises a transistor of the PNP typewith the emitter electrode connected directly to the supply voltageconductor 36. The base electrode is connected through conductor 90 tothe plate of amplifier stage 22 and the collector electrode is connectedthrough the energizing coil 88 of relay 26 to ground. A condenser 94between the base and collector electrodes provides degeneration for thealternating voltage components to prevent oscillation. The connection ofthe collector electrode through resistor 86 to the grid of amplifierstage 22 provides positive feedback which causes amplifier stages 22 and24 to function as a bistable or flip-flop circuit. In the absence oflight, the plate voltage of amplifier stage 20 on the grid of amplifierstage 22 permits current in the emitter to base circuit of thetransistor which permits sufiicient current to flow in the emitter tocollector circuit to energize or pull-in the control relay 26 and causethe movable contact 74 to engage the fixed contact 73. In condition, thecontrol relay is effective to switch the headlamps to the upper beam.

The control relay 26 is adapted to control energization of the powerrelay 28 and is connected therewith through the manual control unit 18.The manual control unit includes a control switch which has a movablecontact 96, and a fixed contact 98 corresponding to automatic control,and a fixed contact 100 which is an open circuit. The fixed contact 73of control relay 26 is connected through a terminal 104 to the fixedcontact 98 of the control switch. The movable contact 96 is connectedthrough the energizing coil 106 of the power relay 28 to the battery 30.Thus when the control relay is energized,

in the absence of light on the photocell, the power relay- 28 isenergized by the battery. The manual switch 18 also includes an overrideswitch 102 which is normally open. The override switch 102 has its fixedcontact connected through a terminal 188 and the conductor 78 to thecathode of amplifier stage 22 and its movable contact connected toground. Thus when the override switch is closed, the amplifier stage 22is caused to be conductive regardless of light on the cell 14 and thecontrol relay is energized.

The power relay 28, having the energizing coil 106, includes a pair offixed contacts 110 and 112 and a movable contact 114. The movablecontact is spring biased into engagement with the fixed contact 118 andis connected with the battery 30. The headlamp circuit 10 includes alower beam filament ,116 connected between the fixed contact and groundand an upper beam filament 118 connected between the fixed contact 112and ground. When the power relay is deenergized, the lower beam filament116 is energized and when the relay is energized, the upper beamfilament 118 is energized.

To consider the operation of the inventive system, assume that there isno light on the photoconductive cell 14. Accordingly, the cell exhibitsa high resistance and the signal voltage across the resistor 52 issubstantially zero and the amplifier stage 20 is essentiallynon-conductive. Since the grid of amplifier stage 22 is held at thevoltage of the plate of amplifier stage 2i and is approximately equal toits cathode voltage, the amplifier stage 22 is conductive. Accordingly,the plate to cathode circuit of amplifier 22 presents a low resistancein the base circuit of the transistor stage 24 and emitter to basecurrent flows. This permits current flow from the emitter to collectorthrough the energizing coil 88 of the control relay 26 and the relay issufficiently energized to cause movable contact '74 to engage the fixedcontact 73. Thus, the power relay 106 is energized from the battery 3t:and the movable contact 114 engages the fixed contact 112 to energizethe upper beam filament 118 of the eadlamp circuit.

With the control relay 26 energized, the sensitivity switching circuit,including potentiometer resistor 40 and conductor 42, is connected toground through fixed contact 73 and movable contact 74. The voltageapplied across the cell 14 is thus determined by the position of themovable contact 44 on potentiometer resistor 49 and the value of signalvoltage across resistor 52 for a given light intensity is thusestablished. The potentiometer resistor 40 is termed the dim sensitivitycontrol and permits adjustment of the value of light intensity at whichthe control relay will drop out or become deenergized.

Consider now the dimming or lower beam switching cycle. As the lightintensity on the photocell T4 is increased, as by the headlamps of anoncoming vehicle, the signal voltage developed across the resistor 52becomes increasingly positive. Consequently, the amplifier stage 20becomes increasingly conductive and the voltage on the plate thereofdiminishes. As a result, the voltage on the grid of amplifier stage 22becomes less positive and reduces the conduction of amplifier stage 22.This decreases the emitter to base current in the transistor stage 24and hence the emitter to collector current through the energizing coil88 of control relay 26 decreases. The decrease of current through theenergizing coil 88 causes its magnetic field to collapse and produce anegative-going voltage which is applied through resistor .86 in aregenerative sense to the grid of amplifier stage 22 to further reducethe grid voltage and drive the amplifier stage 22 to the nonconductivestate to deenergize the relay 26. This causes the movable contact "1'4to engage the fixed contact 72 thereby interrupting the energizingcircuit for the power relay 28. Accordingly, in the power relay, themovable contact 114 engages the fixed contact 110 and energizes thelower beam filament 116 in the headlamp circuit.

The deenergization of the control relay 26 also interrupts thesensitivity switching circuit extending from potentiometer resistor 40through conductor 42 to ground. (Donsequently, the entire voltage onsupply voltage condoctor 36 is applied from the movable contact 44across the photoconductive cell circuit. This increased voltage acrossthe photocell 14 increases the sensitivity of the photocell circuit by afactor of 10 to 12 and therefore the same signal voltage will bedeveloped across load resistor 52 by a light intensity of much lowervalue.

The value of signal voltage applied to the grid of amplifier stage 20when the control relay 26 is deenergized may 'be adjusted by thevariable load resistor 52. Thus the value of light intensity at whichthe control relay 26 will be reenergized may be established byadjustment of the resistor 52 which is termed the hold sensitivitycontrol.

With the control relay 26 deenergized and the lower beam filamentsenergized, the system is in readiness for the return or upper beamswitching cycle. The time constant of the cell 14 is appreciably longerthan the acceptable time constant for automatic headlamp control and toimprove the system performance, the time constant circuit includingcondenser 68 and resistor 64 are provided. When the light intensity onthe cell 14 decreases rapidly, as occurs when an oncoming vehiclepasses, the

signal voltage across the load resistor 52 decreases at a ratedetermined by the time constant of the cell. The time required for thesignal voltage to decrease to the hold value is of the order of onesecond whereas it is desired to return the headlamp circuit to the upperbeam withina time interval of 0.2 to 0.3 second. The condenser 68 isconnected between cathode and ground when the control relay 26 is in thelower beam position and therefore is charged to hold the cathode ofamplifier stage 20 at a positive voltage relative to ground. Upon thesudden decrease of light intensity, the grid voltage "starts to decreaseimmediately but the cathode voltage decreases at a rate determined bythe time constant of condenser 68 and resistor 64. Consequently, thereis a time delay in the voltage change of the cathode preventing it fromtracking the grid voltage and a sufiicient difference betwen the gridand cathode voltages is achieved within the required time interval forheadlamp switching. Since the conduction of amplifier stage 20 isimmediately decreased, the amplifier stage 22 becomes more conductiveand its plate voltage decreases. Accordingly, transistor stage 24becomes increasingly conductive and the increased current flow throughthe coil .88 causes a regenerative voltage to be fed back throughresistor 86 to the grid of amplifier stage 22 to increase .theconductiontherethrough sufiiciently to energize the control relay 26.-

If the time constant circuit, including condenser 68, were connectedduring the dimming cycle, dimming action would result from flashes oflight of less intensity than the dimming level. A sudden increase oflight in- :.tensity, as by a momentary flash, would cause the gridvoltage of amplifier stage 20 to increase rapidly and the condenser 68would tend to prevent the cathode voltage .from increasing therewith andthe amplifier stage 20 would become conductive rapidly. To prevent thisundesirable operation, the time constant circuit, including condenser68, is interrupted during the dimming cycle by .the control relay 26when its movable contact 74 engages ;the fixed contact 73.

' System sensitivity, which may be considered in terms ;of distance fromthe oncoming vehicle at which dimming poems, is provided by the movablecontact 82 on the potentiometer resistor 84. The position of the movable--contact 82, which is accessible to the vehicle driver, deytermines thecathode voltage in amplifier stage 22 and adjustment thereof changes thedim sensitivity and the .hold sensitivity proportionally. Additionally,it is noted .that the resistor 38 and the potentiometer resistor 40 areconnected across the contacts of the control relay to minimize thedestructive eliect of switching the power relay current. The resistor 50in the photoconductive cell circuit limits the current flow therein toprevent the ,grid signal voltage across load resistor 52 from becoming.sufliciently positive to cut-off the plate current of the xamplifierstage 20.

The modification of the circuit as shown in Figure 2,

involves the hold and dim sensitivity controls. The load resistor 52 isa fixed resistor to prevent variation of the grid circuit resistance ofamplifier stage 20 which tends '.'to shift the keying point of theamplifier. Additionally, a filter condenser 120 is connected across thefixed load resistor 52'. The hold sensitivity control is adjusted bychanging the voltage across the photoconductive cell ch cuit and forthis purpose, a potentiometer resistor 40' in series with the resistor38 is connected across the supply voltage conductor 36 and ground. Themovable contact 44' of potentiometer resistor 40' is positioned toestablish the desired value of hold sensitivity. The dim sensitivitycontrol is provided in the sensitivity switching circuit 42 by avariable resistor 122 which is adjusted to establish the desired dimminglevel. Otherwise, the circuit configuration and operation is the same asthat of the circuit in Figure 1.

' Additional modifications of the inventive system are represented bythe circuit of Figure 3. In Figure 3, the energizing circuit for thephotoconductive cell circuit extends from the battery 30 throughconductor 32, terminal 34 to terminal and thence through conductor 60'and the potentiometer resistor 130 to ground. The photoconductive cellcircuit is connected between the movable contact 132 of thepotentiometer resistor 130, through the photoconductive cell 14,terminal 48, current limiting resistor 50 and then in parallel throughvariable load resistor 52 and the sensitivity switching circuit 42'which includes variable resistor 124 and shunt condenser 126. Thevariable load resistor 52 serves as the hold sensitivity control and thevariable resistor 124 serves as the dim sensitivity control. In thisarrangement, the sensitivity switching circuit 42, which is interruptedby the control relay on the dimming cycle, modifies the signal voltageby a change of load resistance in the photoconductive cell circuitrather than by change of voltage across the photoconductive cell circuitas in the circuits of Figures 1 and 2. The system sensitivity in thisarrangement is adjusted by positioning the movable contact 132. ofpotentiometer resistor 130 which alters the voltage across thephotoconductive cell circuit and changes the dim and hold control levelsproportionally.

The speed up of switching on the return or upper beam switching cycle isprovided by the condenser 68 connected across the cathode bias orself-bias resistor 64 and forming a time constant circuit therewith. Inorder to] counteract the effect of the condenser 68' on the dimming orlower beam switching cycle, the condenser 126, forming a time constantcircuit with variable resistor 124, causes a time delay in the change ofgrid voltage in amplifier stage 20 corresponding to the time delay inthe change of cathode voltage developed by the condenser 68f.

An additional modification, shown in the circuit of Figure 3, causes theupper beam headlamp circuit to be energized when the power relay isdeenergized and the lower beam headlamp circuit to be energized when thepower relay is energized. In this arrangement, the upper beam filamentis connected between the fixed contact and ground and the lower beamfilament 116 is connected bet-ween the fixed contact 112 and ground.Additionally, the connections to the control relay are modified so thatthe fixed contact 72 is connected to the power relay energizing coil 106and battery 30 so that the power relay is deenergized when the controlrelay is energized. The fixed contact 73 is connected with thesensitivity switching circuit 42 to connect this circuit to ground whenthe control relay is energized on the upper beam switching cycle. Afixed resistor 128 is connected between the supply voltage conductor 36and a fixed contact 72 of the control relay to minimize the destructiveeffect of current interruption on the contacts of the control relay. Acondenser 94' is connected across the energizing coil 88 of the controlrelay to bypass the alternating voltage components to preventoscillation of the system. Otherwise, the circuit configurations andoperation is the same as described with reference to Figure 1.

Although the description of this invention has been given with respectto a particular embodiment, it is not to be construed in a limitingsense. Numerous variations and modifications within the spirit and scopeof the invention will now occur to those skilled in the art. For

7 a definition of the invention, reference is made to the appendedclaims.

I claim:

1. An automatic control system for vehicle headlamps having upper andlower beam circuits, a light responsive device for developing a signalvoltage and having a time constant longer than that desired forswitching from one beam circuit to the other, an amplifier having aninput circuit including said device and having an output circuit, arelay connected with said outptu circuit for energization thereby andincluding switching means operati 'ely connected with said upper andlower beam circuits for selective energization thereof, said amplifierincluding a biasing means developing a, bias voltage corresponding tothe conduction in its output circuit, and timing means connected to saidbiasing means and to said switching means to delay change of said biasvoltage when one of said beam circuits is-energized, said timing meansbeing disconnected from said biasing means to permit instantaneouschange of said bias voltage when the other of said beam circuits isenergized.

2. An automatic control system for vehicle headlamps having upper andlower beam circuits, a light responsive device for developing a signalvoltage and having a time constant longer than that desired forswitching from one beam circuit to the other, an amplifier having an.input circuit including said device and an output circuit, a relayconnected with said output circuit for energization thereby andincluding switching means operatively connected with said upper andlower beam circuits for selective energization thereof, said amplifierincluding a self-biasing resistor in its input circuit, and a timingcondenser being connected across said resistor through said switchingmeans to delay the chmge of self-bias when one of said beam circuits isenergized and being disconnected from said resistor by said switchingmeans to permit instantaneous change of said self-bias when the other ofsaid beam circuits is energized.

I 3. A light responsive control system comprising a voltage source, aphoto-conductive cell and a load resistor serially connected across thevoltage source, said photo-1 conductive cell having a time constantlonger than that desired for the system, an electron tube having plate,cathode, and grid electrodes, means applying a voltage between the plateand cathode electrodes, a self-biasing resistor and said load resistorbeing connected between the grid and cathode electrodes for supplyingthe signal voltage to the grid corresponding to the photoconductive cellcurrent variations, relay means connected with the plate electrode andresponsive to plate current variations, a condenser connected acrosssaid self-biasing resistor to prevent instantanenous change of voltageof said cathode electrode whereby the grid voltage changes at a greaterrate than the cathode voltage to cause the plate current to change at agreater rate than the photoconductive cell current.

4. A light responsive control system comprising a voltage source, aphotoconductive cell and a load resistor serially connected across thevoltage source, said photoconductive cell having a time constant longerthan that desired for the system, an electron tube having plate,cathode, and grid electrodes, a cathode resistor connected between thecathode electrode and ground, means applying a voltage between the plateelectrode and ground, said load resistor being connected between thegrid electrode and ground for supplying the signal voltage to the gridcorresponding to the photoconductive cell current variations, relaymeans connected with the plate electrode and responsive to plate currentvariations, a condenser connected across said cathode resistor toprevent instantaneous change of voltage of said cathode electrodewhereby the grid voltage changes at a greater rate than the cathodevoltage to cause the plate current to change at a greater rate than thephotoconductive cell current.

5. A light responsive control system comprising a.

voltage source, a photoconductive cell and-a load liesistor seriallyconnected across the voltage source, said photoconductive cell having atime constant longer than that desired for the system, an amplifierhaving input and output circuits andincluding an electron tube havingplate, cathode, and grid electrodes, a self-biasing resistor connectedbetween the cathode electrode and ground, means applying a voltagebetween the plate electrode and. ground, said load resistor beingconnected in said input circuit between the grid electrode and groundfor supplying a signal voltage to the grid corresponding to the photo,-conductive cell current variations, a relay having an energizing coil, amovable contact and a fixed contact, one contact being connected toground and said coil being connected in said output circuit forenergization thereby, a condenser connected between the cathodeelectrode and the other contact whereby the condenser is, connectedacross the self biasing resistor when the contacts are closed to preventinstantaneous change of voltage of said cathode electrode and allow thegrid voltage to change at a greater rate than the cathode voltage tospeed up the change of energization of said relay when the lightintensity changes, said condenser being disconnected from across theself-biasing resistor when the contacts are open whereby the gridvoltage and the cathode voltage change at the same rate.

6. A light responsive control system comprising a voltage source, aphotoconductive cell and a load resistor serially connected across thevoltage source, said photoconductive cell having a time constant longerthan that desired for the system, an amplifier having input and outputcircuits and including an electron tube having plate, cathode, and gridelectrodes, a self-biasing resistor connected between the cathodeelectrode and ground,

means applying a voltage between the plate electrode and ground, saidload resistor being connected in said input circuit between the gridelectrode and ground for supplying a signal voltage to the gridcorresponding to thereby, a first timing condenser connected across saidself-biasing resistor to prevent instantaneous change of voltage of saidcathode electrode and allow the grid voltage to change at a greater ratethan the cathode voltage to speed up the change of energization of saidrelay when the contacts are open, a second timing condenser connectedacross said load resistor through said contacts whereby the grid voltageand the cathode voltage change at the same rate when the contacts areclosed.

7. A light responsive control system comprising a voltage source, aphotoconductive cell and a load resistor serially connected across thevoltage source, said photconductive cell having a time constant longerthan that desired for the system, an amplifier having input and outputcircuits and including an electron tube having plate, cathode, and gridelectrodes, at self-biasing resistor connected between the cathodeelectrode and ground, means applying a voltage between the plateelectrode and ground, said load resistor being connected in said inputcircuit between the grid electrode and ground for supplying a signalvoltage to the grid corresponding to the photoconductive cell currentvariations, a relay having an energizing coil, a movable contact and afixed contact, one contact being connected to ground and said coil beingconnected in said output circuit for energization thereby, a firsttiming condenser connected across said self biasing resistor to preventinstantaneous change of voltage of said cathode electrode and allow thegrid voltage to change at a greater rate than the cathode voltage tospeed up the change of energization of said relay when the contacts areopen, a second timing condenser and an additional load resistorconnected across the first-mentioned load resistor throug said contactswhereby the grid voltage and the cathode voltage change at the same ratewhen the contacts are closed.

8. An automatic control system for vehicle headlamps having upper beamand lower beam circuits, a voltage source, a photoconductive cell and afirst resistor in series connection across the voltage source fordeveloping a signal voltage across the first resistor corresponding tothe light intensity on said cell, a relay including switching meansoperatively connected with said upper and lower beam circuits, anamplifier having an input circuit connected across the first resistorand an output circuit connected with said relay means for causingactuation of the switching means at a predetermined value of lightintensity, a second resistor connected in parallel with said firstresistor through the switching means for changing the value of thesignal voltage corresponding to a given value of light intensity wherebythe relay means actuates said switching means upon the occurrence of adifferent value of light intensity.

, 9. An automatic control system for Vehicle headlamps having upper andlower beam circuits, a light responsive device for developing a signalvoltage corresponding to the intensity of incident light, a controlrelay having switching means operatively connected with said upper andlower beam circuits and having an energizing coil for actuating saidswitching means, a first amplifying device having its input circuitconnected with the light responsive device and being non-conductive whenthere is no light on said light responsive device, a second amplifyingdevice having its input circuit connected with the output circuit of thefirst amplifying device and being conductive when the first amplifyingdevice is non-conductive, a third amplifying device having its inputcircuit connected to the output circuit of the second amplifying deviceand including said energizing coil, said third amplifying device beingconductive when the output circuit of the second amplifying device isconductive, and positive feedback circuit from the output circuit of thethird amplifying device to the input circuit of the second amplifyingdevice whereby a change of signal voltage causes an inverse change ofconduction of the first and third amplifying devices and the feedback tosecond amplifying device promotes rapid change of conductiontherebetween to change the energization of said relay.

10. An automatic control system for vehicle headlamps having upper andlower beam circuits, a light responsive device for developing a signalvoltage corre- 'sponding to the intensity of incident light, a controlrelay having switching means operatively connected with said upper andlower beam circuits and having an energizing coil for operating saidswitching means, a first electron tube having an input circuit extendingbetween its grid and cathode and including said light responsive device,a second electron tube having an input circuit extending between itsgrid and cathode and having its grid connected with the plate of thefirst electron tube, a transistor having an input circuit extendingbetween its base and emitter and connected across the output circuit ofthe second electron tube, said transistor having an output circuitextending between its emitter and collector electrodes and includingsaid energizing coil whereby a change of signal voltage causes aninverse change of conduction in the output circuits of the firstelectron tube and said transistor.

11. An automatic control system for vehicle headlamps having upper andlower beam circuits, a light responsive device for developing a signalvoltage corresponding to the intensity of incident light, a controlrelay having switching means operatively connected with said .upper andlower beam circuits and having an energizing coil for operating saidswitching means, a first electron tube having an input circuit extendingbetween its grid and cathode and including said light responsive device,

a second electron tube having an input circuit extending between itsgrid and cathode and having its grid con- 10 ,nected with the plate ofthe first electron tube, a tr'ari sistor having an input circuitextending between its base and emitter and connected across the outputcircuit of the second electron tube, said transistor having an outputcircuit extending between its emitter and collector electrodes andincluding said energizing coil, and a positive feedback circuit from thecollector of said transistor to the grid of the second electron tubewhereby a change of signal voltage causes an inverse change ofconduction in the output circuits of the first electron tube and thetransistor, and the feedback to the second electron tube promotes arapid change of conduction between the first electron tube and thetransistor output circuits to change the energization of said relay.

12. An automatic control system for vehicle headresponsive device fordeveloping a signal voltage corresponding to the intensity of incidentlight, a control relay having switching means operatively connected withsaid upper and lower beam circuits and having an energizing coil foractuating said switching means, a first electron tube having an inputcircuit extending between its grid and cathode and including said lightresponsive devices and being nonconductive when there is no light onsaid device, a second electron tube having its grid connectedi with theplate of the first electron tube and being con-- ductive when the firstelectron tube is nonconductive, a: transistor having its base connectedwith the plate of": the second electron tube, its emitter connected withsaid"-' voltage source, and its collector connected with said;energizing coil, and a resistive feedback path connected between saidenergizing coil and the grid of said second! electron tube whereby thepositive feedback voltage to: the second electron tube promotes thechange of current through said energizing coil in response tovariations; of said signal voltage.

13. An automatic control system for vehicle head lamps having upper andlower beam circuits, a voltage; source, a photoconductive cell and aload resistor serially: connected with said voltage source fordeveloping a: signal voltage corresponding to the intensity of lightupon said cell, a control relay having switching means operativelyconnected with said upper and lower beam circuits and having anenergizing coil for actuating said switching means, a first electrontube having an input circuit extending between its grid and cathode andincluding said load resistor and having an output circuit extendingbetween its plate and cathode and including a plate resistor and saidvoltage source, said first electron tube being nonconductive in theabsence of light on said cell, a second electron tube having an inputcircuit extending between its grid and cathode and connected across theoutput circuit of the first electron tube and having an output circuitincluding a plate resistor and said voltage source, said second electrontube being conductive when the first electron tube is nonconductive, atransistor hav-. ing an input circuit extending between its emitter and;base and connected across the plate resistor of the sec-, ond electrontube, and having an output circuit extending: between its emitter andcollector circuits and connectedtl across said energizing coil, and aresistive feedback cir cuit connected between said collector and grid ofthe, second electron tube to provide positive feedback there-l betweento rapidly cut off conduction of said second; electron tube and saidtransistor when said signal voltages, reach a predetermined value.

14. An automatic control system for vehicle headlamps having upper andlower beam circuits, a photoconductivet cell and a load resistorserially connected across a volt-. age source for developing a signalvoltage corresponding; to the intensity of incident light, saidphotoconductive cell having a time constant longer than that desiredforthe system, a first electron tube having its grid connected with saidload resistor, its cathode connected to ground"; through a self-biasingresistor and its plate electrode cona endasnected to said voltage sourcethrough a plate resistor, a relay having an energizing coil, a movablecontact and a fixed contact, one contact being connected to ground, acondenser connected between the cathode of the first electron tube andthe other contact whereby the condenser is connected across theself-biasing resistor when the contacts are closed to allowlthe gridvoltage to change at a greater rate than the cathode voltage, a secondelectron tube having its grid connected with the plate of the firstelectron tube, its cathode connected to ground and its plate connectedwith said voltage source through a plate load resistor, a transistorhaving its emitter connected with said voltage source and its baseconnected with the plate of the second electron tube whereby inputcurrent flows in the transistor when the second electron tube isconductive, said transistor having its collector connected through saidenergizing coil to ground whereby said relay is energized in the absenceof light on the photoconductive cell and a positive feedback connectionbetween said coil and the grid of the second electron tube to promoterapid deenergization of said relay when the signal voltage increases toa predetermined value.

l5. An automatic control circuit for vehicle headlamps having upper andlower beam circuits, a light responsive device for developing signalquantity corresponding to incident light intensity and having a timeconstant longer than that desired for switching from one beam circuit tothe other, an amplifier having an input circuit including said deviceand having an output circuit, the current conduction in the outputcircuit being variable in accordance with the signal quantity, a relayconnected with said output circuit for energization thereby andincluding switching means operatively connected with said upper andlower beam circuits for selective energization thereof, bias meansconnected with the am plifier and responsive to current conduction inthe output circuit to develop a bias condition which decreases theeffect of the signal quantity when the light intensity is increasing,and means connected with the bias means for maintaining said biascondition momentarily during a decrease of light intensity whereby thebias condition increases the effect of the signal quantity and causesimmediate actuation of the switching means when the light intensitydecreases.

16. An automatic control circuit for vehicle headlamps having upper andlower beam circuits, a light responsiveidevice for developing signal:quantity corresponding to light intensity and. having a time constantlonger than that desired for' switching from one beam circuit to theother, an amplifier having an input circuit including: said device andhaving an output circuit, the current conduction in the output circuitbeing variable with the signal quantity, a relay connected with saidoutput circuit for emergization thereby and including switching meansoperatively connected with said upper and lower beam circuits forselective emergization thereof, means connected with the amplifier forreducing the sensitivityof the amplifier to change the effect of thesignal quantity when the light intensity changes in one sense, and meansfor maintaining the changed sensitivity momentarily duringa change oflight intensity in the other sense whereby the last-mentioned meansincreases the effect ofthe signal quantity and causes immediateactuation of the switching means when the light intensity changes insaid other sense. a

l7. Anrautomatic control circuit for vehicle headlamps having, upper andlower beam circuits, a light re sponsive device for developing signalquantity corre sponding to incident light intensity and having a timeconstant longer than that desired for switching from one beam circuit tothe other, an amplifier having an input circuit including said deviceand having an output circuit, the current conduction in the outputcircuit being variable in accordance with the signal quantity, a relayconnected with said output circuit for energization there by andincluding switching means operatively connected with said upper andlower beam circuits for selective energization thereof, aresistance-capacitance bias circuit connected with the amplifier andresponsive to current conduction in the output circuit to develop a biascondition which decreases the effect of the signal quantity when thelight intensity is increasing, and means connected with theresistance-capacitance bias circuit for maintaining said bias conditionmomentarily during a decrease of light intensity whereby the biascondition increases the effect of the signal quantity and causesimmediate actuation of the switching means when the light intensitydecreases.

References Cited in the file of this patent UNITED STATES PATENTS2,829,307 Miller et a1 Apr. 1, 1958

